Inhibition of osteoclast formation is a key mechanism by which estrogen prevents bone loss. Studies have shown that the anti-osteoclastogenic effect of E2 results from the ability of the sex steroid to block the production of TNF from bone marrow monocytes. However, the mechanism by which E2 regulates TNF production remains unknown. We have recently discovered that E2 decreases TNF gene expression by blocking the binding of members of the Jun family of transcription factors to the AP-1 site of the TNF promoter. Therefore, we propose to determine if E2 regulates the production Jun factors and/or induces post translational modification of Jun proteins which decrease their ability to bind to the Ap-1 site. This will be accomplished by determing the effects of in vitro E2 treatment on Jun mRNA expression, mRNA transcription rate and message stability. Once the E2 regulated step is identified, we will determine the mechanism by which E2 regulate Jun mRNA transcription and/or stability. We will then investigate if E2 regulate the phosphorylation and the redox status of Jun. These post-translational modifications are both known to regulate Jun binding to the AP-1 site. All of these studies will be carried out in RAW 264.7 cells, a monocytic line, and murine bone marrow monocytes. The second objective of this application is to determine if the inhibitory effects of E2 are mediated by the estrogen receptor (ER) alpha or beta. This will accomplished by in vitro studies with RAW 264.7 cells and murine bone marrow monocytes as well as by analyzing the effect of E2 in monocytes harvested from ERalpha deficient mice. The relevance of the proposed studies is high because identification of the mechanism by which E2 blocks the monocytic production of TNF provides potential therapeutic targets for postmenopausal osteoporosis.